MOSCOW — Alexei Navalny, the fiercest foe of President Putin who crusaded against official corruption and staged massive anti-Kremlin protests, died in prison Friday, Russia’s prison agency said. He was 47.

The Federal Prison Service said in a statement that Navalny felt unwell after a walk on Friday and lost consciousness. An ambulance arrived to try to rehabilitate him, but he died. There was no immediate confirmation of Navalny’s death from his team.

Navalny, who was serving a 19-year sentence on charges of extremism, was moved in December from his former prison in the Vladimir region of central Russia to a “special regime” penal colony — the highest security level of prisons in Russia — above the Artic Circle.

His allies decried the transfer to a colony in the town of Kharp, in the Yamalo-Nenets region about 1,200 miles northeast of Moscow, as yet another attempt to force Navalny into silence.

The remote region is notorious for long and severe winters. Kharp is about 60 miles from Vorkuta, whose coal mines were part of the Soviet gulag prison-camp system.

Earlier this month Navalny had urged Russians to show their protest of President Putin during next month’s presidential balloting by voting at a specific time on election day.

In a social media statement relayed from the Arctic penal colony where he was being held, Navalny had argued that by forming long queues at noon on March 17 to vote against Mr. Putin, people would make “a powerful demonstration of the country’s mood.” He described it as “a real, all-Russia protest action that will take place not just in every city, but in every district of every city.”

Putin is almost certain to win his fifth term in office in the election because of his tight control over the political system, with most opposition politicians either in jail or in exile abroad and the vast majority of independent media in Russia blocked.

Navalny, who rejected the charges against him as politically motivated, until December 2023 was serving time at Penal Colony No. 6 in the Vladimir region east of Moscow, where he spent months in isolation. 

Then he was transferred to a “special regime” penal colony — the highest security level of prisons in Russia — above the Arctic Circle.

In January, long lines formed at the campaign offices of a 60-year-old politician, Boris Nadezhdin, seeking to run against Putin with an antiwar agenda. Scores of people queued up in cities across Russia to sign petitions in support of his candidacy in what many commentators described as a safe way to show defiance and express protest against the Kremlin.

Election authorities subsequently rejected Mr. Nadezhdin as a candidate, a strong signal from the Kremlin that it won’t tolerate any public opposition to the invasion of Ukraine.

Navalny was a fierce critic of Russia’s invasion of Ukraine. “It is now everyone’s duty to make at least some, even the smallest contribution to stop this war and remove Putin from power,” the dissident said on Twitter in 2022. “Protest wherever and however you can. Agitate however you can and whomever you can. Inaction is the worst possible thing. And now its consequence is death.”

In a crackdown on opposition activists and independent journalists, Mr. Navalny’s Anti-Corruption Foundation was in 2021 labeled an extremist organization by the Kremlin. Now operating outside Russia, the group has continued running investigations into government corruption, even with its leader behind bars.

“When you work with Alexei and you spend enough time with the man, you can’t help but be optimistic,” the director of a documentary about Navalny, Daniel Roher, said in 2022. “This war that Putin is waging, the war crimes he’s committing, are perhaps the greatest political blunder ever. I would look to history. I would remind readers how quickly the Soviet Union fell. Things change overnight.”